The present invention relates generally to control systems for phasing a moving web of material to operating machinery located at a fixed operating station along the web and, more particularly, a phasing control system which is adapted to be used in association with a web having repeat length portions which are to be phased to the operating machine which repeat length portions are subject to minor different length variations.
Web phasing systems have long been employed for phasing repeating longitudinal portions of a web having a constant repeat length to operating machinery along the web. For example, a web phasing system is used in a cutterline which cuts carton blanks having printed graphics thereon in order to ensure that the cut made by the cutter device is always made at approximately the same position with respect to the graphics of each repeat length of the web. A phasing device is necessary to ensure that a longitudinal misalignment of the web such as caused by slippage in web conveying rolls, a web splice, or the like, will not cause each of the repeat length portions occurring after such slippage, splice, etc. to be placed out of registry with the operating station machinery. If a significant misregistry of a web repeat length portion and an associated operating machine such as a web cutter does occur, all succeeding portions of the web which are effected by such misregistry must usually by scrapped. Thus, an accurate web phasing device is essential for any commercial high-speed operation in which repeat length portions of a web are operated on at one or more operating stations along the web. To control the phasing of a web with a particular operating station it is necessary to monitor the degree of registry of web repeat length portion with operating station machinery in order to make the necessary adjustments in the web movement or, in some cases, in the operating station machinery movement so as to ensure proper phasing of the web and operating stations. Such monitoring is generally performed by a photoelectric scanning device, generally referred to in the industry as a "photo eye" unit, which senses register marks on the web which are associated with each repeat length portion of the web. In an ideal control situation, the photo eye unit would be positioned within the operating station and would sense a register mark at exactly the time that the associated operation were being performed on the web. For example, in the case of a web carton blank cutting unit, the photo eye would be positioned within the cutter device and would sense a register mark on the web at exactly the same position that the cutter is designed to cut the associated web portion. In such a situation, a cutter position reference signal would also be generated at the time that the cutter was oriented in the cutting position. The cutter position reference signal and the web indicia signal would be compared by associated circuitry or other data processing means such as a computer to determine the degree of misregistry of the web with the cutter. However, in most situations, it is physically impossible to locate a photo eye unit in exactly the correct position within an operating station such that the operating station machinery position reference signal and the indicia sensing signal associated with a repeat length portion of the web being processed will occur at the same time in response to proper registry. In order to approximate a situation in which a web indicia signal will occur at the same instant as an operating station machine reference signal during proper registry, a register mark sensing unit is often placed at a position at an integer number of repeat lengths upstream of an associated operating station, for example, five repeat lengths away. In such a situation, even though the register mark associated with a repeat length which is being operated on by the operating station is not sensed at the same time that a machine reference signal is generated, a register mark which is then positioned beneath the photo eye unit will be sensed at that time, so long as the web repeat length distance remains constant throughout the web. However, a problem with such a sensing device placement system is encountered when web repeat length is subject to variation such as when the web being processed is a relatively extensible plastic film web. In such a situation, even a moderate increase or decrease in the repeat length of the web, e.g. 1/4 inch in a 40 inch repeat length, will completely disrupt phasing control of the web because each succeeding repeat length error between the photo eye unit and operating station will produce an additive misregistry effect. Such misregistry will not be corrected by such a control system due to the erroneous assumption built into the control circuitry that the register mark associated with the subject operating station is located exactly the designed distance, e.g. five, repeat lengths from the register mark associated with the sensing device. To state the problem in a slightly different language, prior art phasing techniques phase a web to a point at an integer number of "ideal" or "design" repeat length distances upstream of an operating station and assume that this will produce proper phasing at the operating station as well. This assumption is incorrect when the actual repeat length distance of the web portions varies from the design repeat length value. The phasing error resulting from this incorrect assumption will be approximately equal to the amount by which the actual repeat length value varies from the ideal repeat length value multiplied by the number of repeat length distances that the photoeye unit is positioned away from the operating machine. To applicant's knowledge, no one in the industry appreciated this phasing problem associated with variable repeat lengths in extensible webs prior to applicant's identification of the problem.
Prior art phasing techniques are also inadequate for dealing with another type of problem encountered with extensible webs. The repeat length distance of extensible webs may vary nonuniformly from repeat length portion to repeat length portion. For example, one repeat length may be 0.1 inches long, the next may be 0.2 inches long, the next may be 0.1 inch short. Prior art techniques control phasing by controlling the position of a repeat length approaching a sensing unit on the phasing error measured in the preceding repeat length portion. Control is achieved by varying the speed of the web in proportion to the measured error. The control assumption underlying this technique is that the phasing error of the next repeat length will be approximately equal to the phasing error in the sensed repeat length. This assumption is invalid for webs having repeat lengths which are subject to variation from repeat length to repeat length and results in phasing error in addition to the phasing error associated with sensing device displacement from the operating station.
A need thus exists for providing a control system for use in phasing an extensible web to an operating machine which adequately accounts for variations in repeat length but which does not require sensing device to be physically located within an operating station at the point where an associated operation is being performed on a web.